Nowadays, R—Fe—B based sintered magnets represented by Nd—Fe—B based sintered magnets have been used in various fields for their high magnetic characteristics. However, an R—Fe—B based sintered magnet contains a highly reactive rare-earth metal: R, and thus is susceptible to oxidization and corrosion in air. Therefore, when such a magnet is used without a surface treatment, corrosion proceeds from the surface due to the presence of small amounts of acids, alkalis, water, etc., whereby rust occurs, causing deterioration or fluctuation in the magnetic characteristics. Further, when such a rusted magnet is incorporated into a device such as a magnetic circuit, the rust may be dispersed and contaminate peripheral parts.
Various methods are known for imparting corrosion resistance to an R—Fe—B based sintered magnet. One of them is a method in which a chemical conversion film containing at least one of Ti and Zr, phosphorus, oxygen, and fluorine as constituent elements is formed over a surface of the magnet with an Al film therebetween (Patent Document 1). This method is recognized by those skilled in the art as a method that allows a magnet to be provided with excellent corrosion resistance. However, with respect to magnets to be incorporated into automotive motors which are used in an environment with wide temperature swings and may also be exposed to chlorine ions contained in an anti-freezing agent spread on the road in a cold region or to seawater near the coast, such magnets are required to have higher corrosion resistance than magnets for domestic use. For this reason, there is a demand for the development of a method for imparting corrosion resistance to a magnet, which is superior to the method of Patent Document 1. Further, when a magnet is incorporated into a part using an adhesive, the magnet is expected to maintain excellent adhesion strength to the adherend even under severe conditions.    Patent Document 1: JP-A-2000-150216